Add section on line breaks to Markdown documentation, update mathematical expressions too (#53662)

Co-authored-by: Jules <19994093+jules-p@users.noreply.github.com>

content/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax.md

@@ -199,6 +199,44 @@ Some text I want to provide a direct link to, but which doesn't have its own hea
 > [!TIP]
 > Custom anchors are not considered by the automatic naming and numbering behavior of automatic heading links.
 
+## Line breaks
+
+If you're writing in issues, pull requests, or discussions in a repository, {% data variables.product.github %} will render a line break automatically:
+
+```markdown
+This example
+Will span two lines
+```
+
+However, if you are writing in an .md file, the example above would render on one line without a line break. To create a line break in an .md file, you will need to include one of the following:
+
+* Include two spaces at the end of the first line.
+  <pre>
+  This example&nbsp;&nbsp;
+  Will span two lines
+  </pre>
+* Include a backslash at the end of the first line.
+
+  ```markdown
+  This example\
+  Will span two lines
+  ```
+
+* Include an HTML single line break tag at the end of the first line.
+
+  ```markdown
+  This example<br/>
+  Will span two lines
+  ```
+
+If you leave a blank line between two lines, both .md files and Markdown in issues, pull requests, and discussions will render the two lines separated by the blank line:
+
+```markdown
+This example
+
+Will have a blank line separating both lines
+```
+
 ## Images
 
 You can display an image by adding <kbd>!</kbd> and wrapping the alt text in `[ ]`. Alt text is a short text equivalent of the information in the image. Then, wrap the link for the image in parentheses `()`.

content/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions.md

@@ -42,12 +42,14 @@ This sentence uses $\` and \`$ delimiters to show math inline: $`\sqrt{3x-1}+(1+
 
 To add a math expression as a block, start a new line and delimit the expression with two dollar symbols `$$`.
 
+>[!TIP] If you're writing in an .md file, you will need to use specific formatting to create a line break, such as ending the line with a backslash as shown in the example below. For more information on line breaks in Markdown, see "[AUTOTITLE](/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#line-breaks)."
+
 ```text
-**The Cauchy-Schwarz Inequality**
+**The Cauchy-Schwarz Inequality**\
 $$\left( \sum_{k=1}^n a_k b_k \right)^2 \leq \left( \sum_{k=1}^n a_k^2 \right) \left( \sum_{k=1}^n b_k^2 \right)$$
 ```
 
-![Screenshot of rendered Markdown showing how a complex equation displays on {% data variables.product.prodname_dotcom %}. The bolded text reads "The Cauchy-Schwarz Inequality". Below the text, there is an equation showing open paren the sum from k equals 1 to n of a sub k b sub k close paren squared is less than or equal to open paren the sum from k equals 1 to n of a sub k squared close paren times open paren the sum from k equals 1 to n of b sub k squared close paren.](/assets/images/help/writing/math-expression-as-a-block-rendering.png)
+![Screenshot of rendered Markdown showing how a complex equation displays on {% data variables.product.prodname_dotcom %}. The bolded text reads "The Cauchy-Schwarz Inequality". Below the text, there is an equation showing the The Cauchy-Schwarz Inequality formula.](/assets/images/help/writing/math-expression-as-a-block-rendering.png)
 
 {% ifversion math-fenced-blocks %}